The invention relates generally to current interruption circuits employing vacuum arc devices for use in controlling fault currents associated with power transmission and distribution lines.
Increased demand for electric power requires utilities to continually enlarge power distribution systems, and to increase the operating voltages of power transmission lines. As the capacity of power systems is enlarged, there is a continuing need in the electric power industry for improved current limiting and interrupting devices.
One type of current interrupting circuit employs a vacuum fault current interrupter which employs separable electrodes in a vacuum enclosure. When the electrodes in such vacuum devices are rapidly separated, arcing occurs in the interelectrode gap. Typically, in prior art vacuum devices of this type, the arc is permitted to burn until a normal current zero in the alternating current cycle, at which point the arc disappears. If sufficient dielectric strength exists in the gap between the contacts, re-ignition is prevented and current interruption is complete.
As the voltages of transmission and distribution lines increase, it becomes increasingly important to interrupt current flow even before the occurrence of a current zero in the alternating current cycle. Fault currents on high voltage lines would otherwise increase so rapidly as to cause significant equipment damage even within the duration of a single current half-cycle. One improved type of vacuum fault current limiting circuit for rapidly extinguishing vacuum arcs is disclosed in U.S. Pat. No. 4,021,628. That patent discloses a current limiter employing a transverse magnetic field of sufficient strength to drive the arc plasma from between the arcing contacts, thus extinguishing the arc. The magnetic field causes an enormous increase in the arc voltage which serves to commute the fault current into a parallel current limiting impedance.
The concept of using a transverse magnetic field, taught in U.S. Pat. No. 4,021,628, offers increased interruption speed and capacity over earlier prior art vacuum interrupters. Nevertheless, commutation failure can occur on such devices when the strength of the transverse field is insufficient to force the arc current to zero. To provide even higher strength magnetic fields in the arc gap of the vacuum device is expensive and presents practical problems in interrupters which must be both highly reliable and rugged. In view of the increasing magnitudes of power carried by power systems, additional suitable techniques are needed to improve the current interrupting effectiveness of such vacuum devices.